The limitations of VB theory

This short section on VB theory is included for historical reasons, and we illustrate the limitations of the VB model by considering octahedral complexes of Cr(III) (d ) and Fe(III) (d ) and octahedral, tetrahedral and square planar complexes of Ni(II) ( / ). The atomic orbitals required for hybridization in an octahedral complex of a first row 

Coordination number Arrangement of donor atoms Orbitals hybridized Hybrid orbital description Example 

Vacant oibitals available to accept ligand electrons 

With the electrons from the ligands included and a hybridization scheme applied for an octahedral complex, the diagram becomes  

This diagram is appropriate for all octahedral Cr(III) complexes because the three 7 d electrons always singly occupy different orbitals. 

For a low-spin octahedral complex such as [Fe(CN)6], we can represent the electronic configuration by means of the following diagram where the electrons shown in red are donated by the ligands  

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